Diastereoselective synthesis of hydroxyethylene dipeptide isosteres

ABSTRACT

A process for the synthesis of hydroxyethylene dipeptide isosteres from α-N,N-di(protected)-amino(alkyl or substituted alkyl)methyl ketones that can be efficiently carried out on an industrial scale. The process proceeds with excellent diastereoselectivity and chemical efficiency, and can be used to prepare a wide variety of hydroxyethylene dipeptide isosteres for a variety of uses, including as HIV-1 protease inhibitors and renin inhibitors.

The U.S. government has rights in this invention by virtue of thepartial funding of work leading to this invention through the NationalInstitutes of Health.

This invention is in the area of organic synthesis, and is in particulara diastereoselective synthesis of hydroxyethylene dipeptide isosteres.

BACKGROUND OF THE INVENTION

Hydroxyethylene dipeptide isosteres ("peptide mimics" or"peptidomimetics," illustrated below) are compounds in which a peptidebond is replaced with a non-hydrolyzable hydroxyethyl isostere thatmimics a peptide enzymic transition state. Compounds incorporatinghdyroxyethyl isosteres have recently generated considerable interest dueto their ability to act as HIV-protease and renin inhibitors. Szelke,M., Jones, D. M., Hallet, A., Leckie, B. J., Proc. Am. Pept. Symp. 8th,1983, 579; Meek, T. D., J. Enz. Inhib., 1992, 6, 65. The amino alcoholfunctionality in active peptidomimetics has (4S,5S) stereochemistry, asindicated below. Peptidomimetics also possess a substituent at the C2position with the indicated absolute configuration. The "S" or "R"designation of the C2-substituent is a function of substituent priority.##STR1##

Large quantities of hydroxyethylene dipeptide HIV-protease inhibitorsand renin inhibitors are currently in demand for laboratory and clinicaltesting as well as for potential commercialization. Many of the priorsynthetic approaches to these isosteres employ the lactone 1 as a keyintermediate which is derivatized via diastereoselective alkylation ofthe enolate followed by ring opening. Several groups have synthesized 1from α-amino aldehydes in a variety of ways, including by: (a) additionof a homoenolate equivalent (DeCamp, A. E., Kawaguchi, A. T., Volante,R. P., Shinkai, I., Tetrahedron Lett., 1991, 32, 1867); (b) addition oflithium ethyl propiolate (Fray, A. H., Kaye, R. L., Kleinman, E. F., J.Org. Chem., 1986, 51, 4828); (c) addition of allylic organometallicreagents (Vara Prasad, J. N. V., Rich, D. H., Tetrahedron Left., 1990,31, 1803); or (d) by conversion of α-amino aldehydes into α-aminoepoxides (Evans, B. E., Rittle, K. E., Homnick, C. F., Springer, J. P.,Hirshfield, J., Veber, D. F., J. Org. Chem., 1985, 50, 4615). Thesynthesis of 1 from a carbohydrate precursor such as D-mannose (Ghosh,A. K., McKee, S. P., Thompson, W. T., J. Org. Chem., 1991, 56, 6500), orvia a γ-ketoester derived from N-Cbz-L-phenylalanine (Hoffman, R. V.,Kim, H., Tetrahedron Lett., 1992, 33, 3579),N-benzyl-N-BOC-phenylalanine (Dondoni, A., et al., Tetrahedron Lett.,1992, 33, 7259), or N-phthalimido-phenylalanine (Sakurai, M., et al.,Tetrahedron Lett., 1993, 34, 5939), have also been reported.

U.S. Patents which disclose methods for the synthesis of hydroxyethylenedipeptide isosteres include U.S. Pat. No. 5,192,668 entitled "Synthesisof Protease Inhibitor," issued Mar. 9, 1993; U.S. Pat. No. 5,188,950entitled "Method of Preparing HIV Protease Inhibitors," issued Feb. 23,1993; U.S. Pat. No. 5,187,074 entitled "Method of Hydroxylation withATCC 55086," issued Feb. 16, 1993; U.S. Pat. No. 5,175,298 entitled"Dipeptide Hydroxy Ethylene Isostere Synthesis and IntermediateTherefor," issued Dec. 29, 1992; and U.S. Pat. No. 5,169,952 entitled"Stereoselective Production of Hydroxyamide Compounds from ChiralAlpha-Amino Epoxides," issued Dec. 8, 1992.

While these syntheses are successful in producing the target compound,the syntheses proceed with variable stereocontrol, and can exhibit oneor more other drawbacks such as a relatively long synthetic sequence,the use of expensive starting materials, or the use of a startingmaterial such as an α-amino aldehyde which is prone to racemization orwhich produces variable diastereoselectivity depending on the nature ofthe "R'" group. For example, the Hoffman process (wherein thehydroxyethylene dipeptide isostere is prepared by the alkylation of at-butyl β-ketoester with an α-bromocarboxylic acid), after hydrolysis,decarboxylation and reduction results in a mixture of 4R and 4S isomersin an approximate ratio of 1.8:1, which must be separated. The DeCamp,et al. process uses an α-amino aldehyde as a starting material that iseasily racemized under a variety of experimental conditions. The aminoaldehyde is reacted with a titanium homoenolate prepared from ethyl3-bromopropionate to provide a mixture of (4S/4R) diastereomers thatmust be separated. The reaction of N,N-dibenzyl-phenylalanine with adichloroisopropoxytitanium homoenolate using the DeCamp protocol resultsin a ratio of 4R to 4S diastereomers of greater than 20 to 1.

Another stereoselective synthesis of hydroxyethylene dipeptide isostereswas recently reported by Diederich and Ryckman. Diederich,"Stereoselective Synthesis of a Hydroxyethylene Dipeptide Isostere,"Tetrahedron Lett., 1993, 34, 6169-6172. The Diederich synthesis is basedon the conversion of a dibenzyl-L-amino acid to the correspondingN'-methyl-O-methylcarboxamide, which is reacted with a Grignard reagentderived from 2-(2-bromoethyl)-1,3-dioxolane or2-(2-bromoethyl)-1,3-dioxane to produce a(2S)-2-dibenzylamino-5-[1,3]dioxolan-2-yl-1-phenyl-pentan-3-one or(2S)-2-dibenzylamino-5-[1,3]dioxan-2-yl-phenyl-pentan-3-one,respectively. Reduction of the carbonyl moieties provides the desiredamino alcohol function with high 4S-stereoselectivity. TheC2-substituent is added by conversion of(2S)-2-dibenzylamino-5-[1,3]dioxolan-2-yl-1-phenyl-pentan-3-one or(2S)-2-dibenzylamino-5-[1,3]dioxan-2-yl-phenyl-pentan-3-one to itscorresponding lactone, followed by alkylation of the lactone and ringopening. Diederich's synthesis suffers from the disadvantages that notall of the reagents are commercially available (the Grignard reagentshave to be generated), the reagents can be relatively expensive, and theoxidation step requires chromium, which presents waste disposalproblems.

None of the known syntheses for hydroxyethylene dipeptide isosteresprovides the optimal combination of the use of stable and inexpensivestarting materials, high stereoselectivity, high yield, and minimalnumber of process steps. In light of the strong need for largequantities of hydroxyethylene dipeptide isosteres for the research anddevelopment of HIV-protease inhibitors and renin inhibitors, it would beof benefit to provide an economical method for their synthesis.

Therefore, it is an object of the present invention to provide a methodfor the preparation of hydroxyethylene dipeptide isosteres that resultsin a product with (4S,5S) stereochemistry.

It is another object of the present invention to provide a method forthe preparation of hydroxyethylene dipeptide isosteres that places asubstituent group in the C2-position with the proper configuration.

It is another object of the present invention to provide a method forthe preparation of hydroxyethylene dipeptide isosteres that is simpleand efficient.

It is another object of the present invention to provide a method forthe preparation of hydroxyethylene dipeptide isosteres that can becarried out on a manufacturing scale.

SUMMARY OF THE INVENTION

A process is provided for the diastereoselective synthesis ofhydroxyethylene dipeptide isosteres from α-N,N-di(protected)amino(alkylor substituted alkyl) methyl ketones that can be efficiently carried outon an industrial scale. This process is a significant advance over priorknown processes for the preparation of this family of compounds, in thatthe process involves only a small number of steps, the startingmaterial, α-N,N-di(protected)amino(alkyl or substituted alkyl) methylketone is less prone to racemization than the prior used aldehydes, andthe overall sequence provides excellent diastereoselectivity andchemical efficiency.

The starting α-N,N-di(protected)amino(alkyl or substituted alkyl)methylketones can be synthesized according to known procedures. As shown inFIGS. 1 and 2, the ketones are converted into their corresponding γ-ketoesters or amides in high yield by treatment with an enolate formingreagent, followed by addition of an α-(leaving group)-acetate orα-(leaving group)-acetamide. The γ-keto ester or amide is then reducedto provide a hydroxyethylene dipeptide precursor with the necessary(4S,5S) stereochemistry.

The important C2-substituent of the hydroxyethylene dipeptide is addedby cyclization of the C2-unsubstituted hydroxyethylene dipeptide to itscorresponding lactone which is alkylated via its enolate. The lactone isthen opened to either a carboxylic acid or amide in good yield. Theester can be converted to the amide according to known procedures.

In an alternative embodiment, as shown in FIG. 3, the enolate of anacetic acid ester, amide or the equivalent, is reacted with anα-N,N-di(protected)amino(alkyl or substituted alkyl) methyl(leavinggroup)ketone to produce the corresponding γ-keto ester, which is treatedas described above to produce the peptidomimetic.

In yet another embodiment, illustrated in FIGS. 1 and 3, theC2-substituent is positioned on the starting acetic acid ester or amide.

Using this method, a wide variety of hydroxyethylene dipeptide isosterescan be prepared for a variety of uses, including as HIV-1 proteaseinhibitors and renin inhibitors.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of a method for preparing ahydroxyethylene dipeptide isostere from α-N,N-di(protected)amino(alkylor substituted alkyl) methyl ketone and an α-(leaving group)-acetateaccording to the present invention.

FIG. 2 is a schematic illustration of one method for preparing apeptidomimetic with an alanine or phenylalanine amino-terminus using thefollowing reagents and conditions: a) NaHMDS, THF, -78° C., b) BrCH₂ CO₂t-Bu, -78° C., c) NAH₄, methanol, 0° C., d) toluene, acetic acid,reflux, e) H₂ /Pd black, absolute ethanol, BOC₂ O, f) phenylCH₂ Br, -78°C., 30 minutes.

FIG. 3 is a schematic illustration of a method for preparing ahydroxyethylene dipeptide isostere from the enolate of an acetic acidester and an α-N,N-di(protected)amino(alkyl or substituted alkyl)methyl(leaving group) ketone according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The term alkyl, as used herein, unless otherwise specified, refers to asaturated straight, branched, or cyclic, primary, secondary, or tertiaryhydrocarbon of C₁ to C₁₀, and specifically includes methyl, ethyl,propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, cyclopentyl,isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl,3-methylpentyl, 2,2-dimethylbutyl, and 2,3-dimethylbutyl. The alkylgroup can be optionally substituted with one or more moieties selectedfrom the group consisting of hydroxyl, amino, alkylamino, arylamino,alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid,phosphate, or phosphonate, either unprotected, or protected asnecessary, as known to those skilled in the art, for example, as taughtin Greene, et al, "Protective Groups in Organic Synthesis," John Wileyand Sons, Second Edition, 1991.

The term alkylamino or arylamino refers to an amino group that has oneor two alkyl or aryl substituents, respectively.

The term "protected" as used herein and unless otherwise defined refersto a group that is added to an oxygen or nitrogen atom to prevent itsfurther reaction during the course of derivatization of other moietiesin the molecule in which the oxygen or nitrogen is located. A widevariety of oxygen and nitrogen protecting groups are known to thoseskilled in the art of organic synthesis.

The term amino acid as used herein, refers to a natural or syntheticamino acid, and includes, but is not limited to alanyl, valinyl,leucinyl, isoleucinyl, prolinyl, phenylalaninyl, tryptophanyl,methioninyl, glycinyl, serinyl, threoninyl, cysteinyl, tyrosinyl,asparaginyl, glutaminyl, aspartoyl, glutaoyl, lysinyl, argininyl, andhistidinyl.

The term aryl, as used herein, and unless otherwise specified, refers tophenyl, biphenyl, or naphthyl, and preferably phenyl. The aryl group canbe optionally substituted with one or more moieties selected from thegroup consisting of hydroxyl, amino, alkylamino, arylamino, alkoxy,aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid,phosphate, or phosphonate, either unprotected, or protected asnecessary, as known to those skilled in the art, for example, as taughtin Greene, et al., "Protective Groups in Organic Synthesis," John Wileyand Sons, Second Edition, 1991.

The term halo, as used herein, includes chloro, bromo, iodo, and fluoro.

The term heteroaryl or heteroaromatic, as used herein, refers to anaromatic moiety that includes at least one sulfur, oxygen, or nitrogenin the aromatic ring. Nonlimiting examples are furyl, pyridyl,pyrimidyl, thienyl, isothiazolyl, imidazolyl, tetrazolyl, pyrazinyl,benzofuranyl, benzothiophenyl, quinolyl, isoquinolyl, benzothienyl,isobenzofuryl, pyrazolyl, indolyl, isoindolyl, benzimidazolyl, purinyl,carbozolyl, oxazolyl, thiazolyl, isothiazolyl, 1,2,4-thiadiazolyl,isooxazolyl, pyrrolyl, quinazolinyl, pyridazinyl, pyrazinyl, cinnolinyl,phthalazinyl, quinoxalinyl, xanthinyl, hypoxanthinyl, pteridinyl,5-azacytidinyl, 5-azauracilyl, triazolopyridinyl, imidazolopyridinyl,pyrrolopyrimidinyl, pyrazolopyrimidinyl, adenine, N⁶ -alkylpurines, N⁶-acylpurines (wherein acyl is C(O)(alkyl, aryl, alkaryl, or aralkyl), N⁶-benzylpurine, N⁶ -halopurine, N⁶ -vinylpurine, N⁶ -acetylenic purine,N⁶ -acyl purine, N⁶ -hydroxyalkyl purine, N⁶ -thioalkyl purine, thymine,cytosine, 6-azapyrimidine, 2-mercaptopyrimidine, uracil, N⁵-alkylpyrimidines, N⁵ -benzylpyrimidines, N⁵ -halopyrimidines, N⁵-vinylpyrimidine, N⁵ -acetylenic pyrimidine, N⁵ -acyl pyrimidine, N⁵-hydroxyalkyl purine, and N⁶ -thioalkyl purine, and isoxazolyl.Functional oxygen and nitrogen groups on the heterocyclic base can beprotected as necessary or desired during the reaction sequence. Suitableprotecting groups are well known to those skilled in the art, andinclude trimethylsilyl, dimethylhexylsilyl, t-butyldimethylsilyl, andt-butyldiphenylsilyl, trityl, alkyl groups, acyl groups such as acetyland propionyl, methylsulfonyl, and p-toluylsulfonyl.

The term alkylheterocyclic or alkylheteroaromatic refers to a moiety inwhich the alkyl group is covalently attached to the heteroaromatic, ispreferably C₁ to C₄ alkyl-heteroaromatic, and more preferably CH₂-heteroaromatic.

The term alkaryl, as used herein, refers to an alkyl group with an arylsubstituent, for example, benzyl, α-methyl benzyl, and phenethyl.

The term aralkyl, as used herein, refers to an aryl group with an alkylsubstituent.

The term alkoxy, as used herein, and unless otherwise specified, refersto a moiety of the structure --O-alkyl.

The term BOC, as used herein, refers to t-butyloxycarboxy.

The term leaving group, as used herein, refers to a moiety that can bedisplaced by an enolate or other nucleophile in an S_(N) 2 reaction.

The term peptide, as used herein, refers to two or more amino acidsconnected via amide linkages.

A process is presented for the preparation of hydroxyethylene dipeptideisosteres in which a peptide bond is replaced with a non-hydrolyzablehydroxyethylene isostere, that mimics a peptide enzymic transitionstate. The process, or standard modifications or extensions thereof, canbe used to prepare a wide variety of HIV-1 protease inhibitors and renininhibitors, including those described in Meek, "Inhibitors of HIV-1Protease, Enzyme Inhibition, 1992, Vol 6., 65-98, incorporated herein byreference.

I. Starting Materials and Intermediates of the Process

In one embodiment, the process, as illustrated in FIG. 1, includesreacting an enolate of an α-N,N-(diprotected)amino ketone of the formulaR¹ CHN(R²)₂ C(O)CH₃ with an α-substituted carboxylic acid ester of theformula CH₂ XC(O)OR³ or R⁴ CHXC(O)OR³, to form a5-substituted-(5S)-5-(N,N-diprotected amino)-4-oxopentanoic acid esterof the formula R¹ CHN(R²)₂ C(O)CH₂ CH₂ C(O)OR³ or R¹ CHN(R²)₂ C(O)CH₂CHR⁴ C(O)OR³, respectively. The latter compound, R¹ CHN(R²)₂ C(O)CH₂CHR⁴ C(O)OR³, on deprotection and reduction of the ketone, provides aselected peptidomimetic with the appropriate (S)-stereochemistry at the4 and 5 positions, and the R⁴ substituent in the C2-position. The formercompound, R¹ CHN(R²)₂ C(O)CH₂ CH₂ C(O)OR³, can be modified to include adesired substituent (referred to as R⁵) in the C2-position by reductionof the C4 ketone followed by cyclization to the lactone followed byalkylation via the corresponding enolate, and ring opening. PreferredC2-substituents are benzyl, isobutyl, methyl, isopropyl, cyclohexyl,t-butyl, aryl, and t-alkyl, for example, t-butyl.

When racemic R⁴ CHXC(O)OR³ is used as a starting material, adiastereomeric product is obtained that can be separated if desiredaccording to known methods. If enantiomerically pure R⁴ CHXC(O)OR³ isused as a starting material, a single enantiomer is produced.

According to the invention, R¹ can be the residue of an amino acid (andpreferably a naturally occurring amino acid), i.e., the moiety connectedto --CHNH₂ CO₂ H, including but not limited to methyl, ethyl, benzyl,hydrogen, isopropyl, HOCH₂ --, --CHOHCH₃, --CH₂ SH, 2-methylpropyl,1-methylpropyl, --CH₂ CH₂ SCH₃, --CH₂ (indole), --CH₂ (p-hydroxyphenyl),--(CH₂)₄ NH₂, --CH₂ (imidazole), --CH₂ CH₂ C(O)NH₂, --CH₂ C(O)NH₂, --CH₂CO² H, --CH₂ CH₂ CO₂ H, (wherein functional groups are protected asnecessary during the reaction), or other alkyl, aryl, alkaryl,heteroaromatic, alkyl(heteroaromatic), arylamino, or aralkyl group. TheR¹ moiety should not render the hydrogen on the adjacent carbon acidicand should not contain a carbonyl moiety that would interfere with thereaction. R¹ is preferably methyl or benzyl.

R² is a bulky group that can control the facial selectivity of attack ofthe enolate on the CH₂ XC(O)OR³ or R⁴ CHXC(O)OR³. R², which can varyindependently on the amine, is preferably a benzyl or substituted benzylgroup, wherein the substituent is alkoxy, preferably p-methoxy. In apreferred embodiment, an R² group is selected that is removed easilyfrom the product of reaction. p-Methoxybenzyl groups can be removed fromthe amine when desired by oxidation with ceric ammonium nitrate.Tertiary butyl groups can be used to direct the facial selectivity ofattack of the enolate, however, they are typically difficult to removefrom the product. In a preferred embodiment, the R² groups on the amineare alike, however, two different R² groups can be used to protect anamine. For example, the amine can be protected with a combination ofbenzyl, BOC, benzyloxycarbonyl, or p-methoxybenzyl, however, in apreferred embodiment, at least one of the groups is a benzyl group. Anappropriate combination of protecting groups can be selected such thatone of the groups can be removed selectively as desired.

R³ can be any moiety that does not adversely affect the S_(N) 2 reactionprotocol, and in particular, which does not encourage attack by theenolate of the α-N,N-(diprotected)amino ketone on the carbonyl of CH₂XC(O)OR³ or R⁴ CHXC(O)OR³. Specifically, R³ can be an alkyl, aryl,heteroaromatic, alkylheteroaromatic, aralkyl, alkaryl, and is preferablya bulky group such as t-butyl which hinders addition to the carbonyl bythe enolate. It has been noted that when ethyl is the ester moiety usingthe reaction protocol set out in FIG. 1 using an ethyl-α-bromo-acetatesubstrate, a complex mixture of products results.

R⁴ is any group that does not unacceptably slow the rate of, orinterfere with the S_(N) 2 reaction, for example, a primary, secondary,or tertiary alkyl or alkaryl group, and is most preferably methyl,ethyl, propyl, isopropyl, butyl, isobutyl, cyclohexylmethyl, benzyl orphenethyl. R⁴ can also be an aryl or aralkyl group, for example, phenylor alkylphenyl, or a heterocyclic or alkylheterocyclic moiety. In oneembodiment, R⁴ is t-butyl.

X is any moiety that can be displaced in an S_(N) 2 reaction, includingbut not limited to bromo, chloro, iodo, triflate, tosylate, diazoniumsalts, mesylates, and brosylates.

R⁵ is any group, which, when attached to X, can be attacked by anenolate ion, resulting in the substitution of the enolate moiety for X,and is typically a primary or secondary alkyl or alkaryl moiety, and ispreferably methyl, benzyl, isobutyl, isopropyl, or cyclohexylmethyl.

It is important to note that either R⁴ or R⁵ becomes the C2-substituentin the hydroxyethylene dipeptide isostere product. According the processdescribed herein, while R⁵ is typically limited to primary or secondaryalkyl or alkaryl groups because it must be susceptible to attack by anenolate ion in an S_(N) 2 reaction, R⁴ can be a wide variety ofmoieties, including R⁵ moieties as well as aryl, heteroaryl and tertiaryalkyl groups.

In an alternative embodiment, as shown in FIG. 3, the enolate of anacetic acid ester formed from CH₃ C(O)O³ or R⁴ CH₂ C(O)OR³ is reactedwith R¹ CHN(R²)₂ C(O)CH₂ X to form R¹ CHN(R²)₂ C(O)CH₂ CH₂ C(O)OR³ or R¹CHN(R²)₂ C(O)CH₂ CHR⁴ C(O)OR³, respectively, which is treated asdescribed above to produce the desired peptidomimetic.

In another alternative embodiment, CH₂ XC(O)N(R⁶)₂, R⁴ CHXC(O)N(R⁶)₂,CH₃ C(O)N(R⁶)₂ or R⁴ CH₂ C(O)N(R⁶)₂, wherein R⁶ is hydrogen, alkyl,alkaryl, aryl, aralkyl, heterocyclic, or alkylheterocyclic, and whereinR⁶ can vary within the molecule, is used in the reaction sequence inplace of the acetic acid ester. For example, a (4S,5S)-hydroxyethylenedipeptide isostere can be prepared by reacting anα-N,N-di(protected)amino methyl(leaving group) ketone with a compoundselected from the group consisting of the enolate of acetic acid esterand and the enolate of acetamide to form the corresponding γ-keto ester,which is further derivatized as described in detail herein.

Some of the compound intermediates used in the disclosed process arenovel compounds (wherein R¹, R², R³, and R⁴ are as defined above unlessindicated otherwise), including but not limited to:

(i) R¹ CHN(R²)₂ C(O)CH₂ CHR⁴ C(O)OR³, wherein R⁴ is as defined above;

(ii) ##STR2## wherein R⁴ is aryl, aralkyl, heteroaryl or t-alkyl;

(iii) ##STR3##

(iv) ##STR4## wherein R₂ and R⁶ are independently (and can vary withinthe molecule) as defined above or hydrogen; and

(v) R¹ CHN(H²)₂ C(O)CH₂ CHR⁴ C(O)OH, wherein R⁴ is as defined above.

Compounds (i) and (v) can be used as intermediates to prepare compoundsthat have a second moiety on the carbon alpha to the carbonyl viaappropriate enolate reactions using Compounds (ii). Compounds (iii) canbe used to prepare Compounds (iv), which can be used in vitro asresearch tools to study the structure-activity relationship, includingbulk tolerance relationships, of HIV-protease inhibitors and renininhibitors, or which can be used in vivo as HIV-protease inhibitors andrenin inhibitors which are administered as disclosed in U.S. Pat. No.5,244,910 entitled "Renin Inhibitors," issued Sep. 14, 1993; U.S. Pat.No. 4,894,437 entitled "Novel Renin Inhibiting Polypeptide AnalogsContaining S-Aryl-D- or L- or DL-cysteinyl, 3-(Arylthio)Lactic Acid or3-(Arylthio)Alkyl Moieties," issued Jan. 16, 1990; U.S. Pat. No.4,882,420 entitled "Dihalo-Statine Substituted Renin Inhibitors," issuedNov. 21, 1989; U.S. Pat. No. 4,880,781 entitled "Renin InhibitoryPeptides Containing an N-Alkyl-Histidine Moiety," issued Nov. 14, 1989;U.S. Pat. No. 4,864,017 entitled "Novel Renin Inhibiting Peptides Havinga Dihydroxyethylene Isostere Transition State Insert," issued Sep. 5,1989; and U.S. Pat. No. 4,705,846 entitled "Novel Renin InhibitingPeptides having a Gamma Lactam Pseudo Dipeptide Insert," issued Nov. 10,1987.

II. Description of Process Steps

The process steps for preparing hydroxyethylene dipeptide isosteres aredescribed in more detail below. Given this disclosure, one of ordinaryskill in the art will be able to routinely modify the synthetic steps asdesired or necessary to achieve specific results. Modifications of theprocess steps or equivalent steps are considered to fall within thescope of this invention.

All of the reactions described below can be carried out in standardorganic solvents that do not react or otherwise interfere with theprocess. Given the description of the process steps below, one ofordinary skill in the art of organic synthesis will be able to select anappropriate solvent for each step. Preferred solvents are typicallytetrahydrofuran and acetonitrile. Other solvents that can be consideredfor selected reactions include a dialkyl formamide, such as dimethylformamide; dialkyl sulfoxide such as dimethyl sulfoxide; chlorinatedsolvents such as dichloromethane, chloroform, carbon tetrachloride,trichloroethane, and tetrachloroethane; alcohols such as methanol,ethanol, and propanol; benzene, alkylated benzenes such as toluene, o,m, and p-xylene; alkoxybenzenes such as o, m, and p-cresol; ethers suchas methyl t-butyl ether, tetrahydrofuran, and diethyl ether; glymes suchas diglyme and triglyme; straight chain or branched alkyl solvents suchas hexane, hexanes, heptane, pentane, and petroleum ether (ligroin);alkyl nitriles other than acetonitrile; nitroalkyl solvents such asnitromethane; and perfluoroalkyl solvents.

Step 1 Preparation of α-N,N-Di(protected)amino(alkyl or substitutedalkyl) Methyl Ketone

α-N,N-Di(protected)amino(alkyl or substituted alkyl) methyl ketones canbe prepared as described in Lagu, et al, "Highly DiastereoselectiveAldol Reactions of Chiral Methyl Ketones," J. Org. Chem., 1993, 58,4191-4193, or Reetz, et al., Tetrahedron: Asymmetry, 1992, 1, 375.Generally, a selected amino acid, for example, a naturally occurringamino acid, is first converted to its N,N-diprotected derivative usingknown procedures. The protecting group typically also adds to thecarboxylic acid moiety, forming an ester that can be removed byselective hydrolysis or selective catalytic transfer hydrogenolysis(Bajwa, Tetrahedron Lett., 1992, 33, 2299). The amino-protectedcarboxylic acid can be converted to the desired ketone using publishedor otherwise known procedures, for example, the Mukaiyama orJorgenson-Gilman protocols (Mukaiyama, et al., Chem. Lett. 1974, 663;Jorgenson, J. Org. React. 1970, 18, 1)

Reetz, et al., synthesized α-N,N-dibenzyl amino ketones from thecorresponding α-(L)-amino acids. They reported that the nitrogenprotecting group on the α-(L)-amino acid has an influence on thediastereoselectivity of nucleophilic addition. Reetz also reported thatthe keto group can be reduced with excellent diastereoselectively toyield a vicinal amino alcohol with S,S stereochemistry at thestereogenic centers. Reetz, M. T., Drews, M. W., Lennick, K., Schmitz,A., Holdgrun, X, Tetrahedron: Asymmetry, 1990, 1, 375; Reetz, M. T.,Drews, M. W., Matthews, B. R., Lennick, J., J. Chem. Soc., Chem.Commun., 1989, 1474.

Lagu, et al, has reported that aldol reactions of lithium enolates ofα-(N,N-dibenzylamino)alkyl methyl ketones proceed diastereoselectivelywith a variety of aldehydes. Neither Reetz nor Lagu extended their studyto the use of α-(N,N-diprotected)alkyl methyl ketones in S_(N) 2displacement reactions.

Step Generation of the Enolate of α-N,N-Di(protected)amino(alkyl orsubstituted alkyl) Methyl Ketone (FIG. 1) or the Enolate of Acetic AcidEster (FIG. 3) and Preparation of γ-keto ester

In the second step according to this process,α-N,N-di(protected)amino(alkyl or substituted alkyl) methyl ketone(FIG. 1) or a selected acetic acid ester or other carboxylic acid ester(FIG. 3) or the corresponding amide is converted to its correspondingenolate, which is reacted with an α-(leaving group) acetic acid ester orα-N,N-di(protected)amino(alkyl or substituted alkyl) methyl(leavinggroup) ketone, respectively, in an S_(N) 2 process to provide a desiredγ-keto ester.

It should be noted that α-N,N-di(protected)amino(alkyl or substitutedalkyl) methyl ketone can form either a thermodynamic (most highlysubstituted) enolate or a kinetic (least substituted) enolate. In thisprocess, it is necessary to generate the kinetic enolate of the methylketone. In contrast, the acetic acid ester can only form one type ofenolate because the carbonyl is flanked by an --OR³ moiety on one side.

Lithium or sodium salts of hindered nitrogen bases are often used toform enolates. A variety of groups can be used to hinder the amine inthe hindered nitrogen base, as known to those skilled in the art.Preferred reagents to generate enolates include amide bases such as LDA,sodium hexamethyldisilazide (NaHMDS), and potassium hexamethyldisilazide(KHMDS), optionally in combination with HMPA, TMEDA, or other reagents,as known to those skilled in the art. Sodium hexamethyldisilazide is apreferred reagent for the formation of the enolate ofα-N,N-di(protected)amino(alkyl or substituted alkyl) methyl ketone inthis process. The sodium enolate produced by sodium hexamethyldisilazideis more ionic, and less regiostable, than its lithium counterpart. Theuse of lithium enolates also results in slightly lower yields in certaincases.

Other enolate forming reagents that can be used in this process includehydride sources such as KH or NaH alone, or trialkyl silyl hydrides withCo₂ (CO)₈, titanium enolate forming reagents, for example, TiCl₄ andboron enolate forming reagents, for example from Bu₂ BOTf.

In one embodiment, a enolate is generated in a derivative of the aceticacid ester or amide in which the moiety attached to the carbonyl is achiral auxiliary, as illustrated below. A wide variety of auxiliariesare known to those skilled in the art, as discussed, for example, inHeathcock, C. H., Modern Synthetic Methods; Scheffold, R., Ed.; VerlagHelvetica Chimica Acta: CH-400 Basel, 1992; pages 1-102; and Evans, D.A., Asymmetric Synthesis, Vol. 3. Academic Press, New York, 1984,Chapter 1. Chiral auxiliaries can simultaneously control both theenolate geometry and the facial selectivity of its reactions withelectrophiles. This provides a means of controlling the stereochemistryat the C2 carbon. Many chiral auxiliaries are imides. Other examples ofcommonly used auxiliaries are 4-methyl-5-phenyl-2-oxazolidinone;4-benzyl-2-oxazolidinone; 4-isopropyl-2-oxazalidinone, and2,10-camphorsultam. ##STR5##

The enolate is typically generated in situ in an inert solvent such astetrahydrofuran under an inert atmosphere such as argon or nitrogen.Other suitable solvents include other ethereal solvents such as diethylether and hydrocarbon solvents such as pentane and toluene. Theα-(leaving group) acetic acid ester or α-N,N-di(protected)-amino(alkylor substituted alkyl) methyl(leaving group) ketone is added slowly tothe reaction solution. This reaction is typically carried out at atemperature ranging between -78 and 0 degrees centigrade, for a timeranging from approximately 30 minutes to two hours, or until completionof reaction.

Step 3 Reduction of γ-Keto Ester to a Hydroxyethylene Dipeptide Isostere

In Step 3, the γ-keto ester prepared in Step 2 is reduced to thecorresponding hydroxyethylene dipeptide isostere using known procedures.A preferred reducing agent is sodium borohydride. Other suitablereducing agents include diisobutylaluminum hydride (DIBAL-H), lithiumborohydride (LiBH₄), and sodium bis(2-methoxyethoxy)-aluminum hydride(Red-Al). The reduction is typically carried out at low temperature, forexample, 0 degrees centigrade, for one hour, or until the reduction iscomplete. The reaction can be conducted in anhydrous methanol or othersuitable solvent.

Step 4 Cyclization of Hydroxyethylene Dipeptide Isostere

Active hydroxyethylene dipeptide isosteres have a C2-substituent. Alkyland alkaryl groups can be added to the C2-position of a C2-unsubstitutedhydroxyethylene dipeptide isostere by cyclization of the isostere to thecorresponding lactone, followed by alkylation of the lactone through itsenolate and ring opening.

Cyclization of the γ-hydroxy carboxylic acid ester can be accomplishedunder acidic conditions as known to those skilled in the art, includingby treatment with methanesulfonic acid and glacial acetic acid in drytoluene. Other acids that can be used in the cyclization processinclude, but are not limited to, sulfuric acid and resinous sulfonicacids, such as Dowex-50 and Nafion, alkylsulfonic acids other thanmethanesulfonic acid, arylsulfonic acids, hydrobromic acid, hydrochloricacid, phosphoric acid, alkylphosphoric acid, nitric acid, nitrous acid,carboxylic acids and diacids, for example as acetic acid, formic acidand oxalic acid. The cyclization is typically conducted under anhydrousconditions at 80-111 degrees Centigrade for six to twelve hours, oruntil the cyclization is complete.

The process embodiment wherein R⁴ CHXC(O)OR³ or R⁴ CH₂ C(O)OR³ is usedas a starting material to form R¹ CHN(R²)₂ C(O)CH₂ CHR⁴ C(O)OR³ obviatesthe need to carry out Steps 4, 5 and 6, wherein the C2-substituent (R⁵)is added to the hydroxypeptide isostere via an enolate reaction.

Step 5 Alkylation of Lactone

The lactone prepared as described in Step 4 or as otherwise known in theart is alkylated through its enolate according to known procedures. Theenolate can be generated in an inert solvent under an inert atmosphereas described in Step 2. A preferred base for the generation of theenolate is sodium hexamethyldisilazide. The enolate is typicallygenerated under anhydrous conditions at a temperature ranging fromapproximately -78 to -60 degrees Centigrade. After generation of theenolate, a selected XR⁵ is added to the reaction solution. The reactionis allowed to proceed to completion, generally in thirty minutes to onehour.

Step 6 Ring Opening of Lactone

The alkylated lactone prepared as described in Step 5 or as otherwiseknown in the art can then be converted into the required isostere by aring opening reaction using the Weinreb amidation protocol. Basha, A.,Lipton, M., Weinreb, S. M., Tetrahedron Lett., 1977, 4171.Alternatively, the lactone can be opened to the corresponding carboxylicacid, that can be converted to the desired isostere using knownprocedures.

Examples 1-7 provide a detailed description for one process for thepreparation of the peptidomimetic(5S)-5-(N,N-dibenzylamino)-2-ethyl-(4S)-4-hydroxy-hexanamide accordingto the present invention, illustrated in FIG. 2. This example is merelyillustrative, and not intended to limit the scope of the invention.

EXAMPLE 1 Preparation of N,N-dibenzylalanine ##STR6##

Benzaldehyde (7.0 mmol, 7.0 mL) was added to a suspension of L-alanine(20.0 mmol, 1.78 g) in acetonitrile (30.0 mL) and water (20.0 mL) atroom temperature. The resulting turbid solution was stirred at roomtemperature for 30 minutes. Sodium cyanoborohydride (2.5 eq., 50.0 mmol,3.2 g) was then added in one portion, and the resultant yellowishsolution was stirred for 60 minutes (exothermic reaction). A few dropsof glacial acetic acid were added in order to maintain a pH ofapproximately 6. After 30 minutes, a white precipitate appeared. Thesuspension was filtered through a sintered glass funnel using 100 mL ofdiethyl ether. The organic solvents were removed in vacuo. The aqueoussolution was extracted with Et₂ O (2×75 mL), and the organic layer waswashed with brine. The organic layer was then dried over anhydroussodium sulfate, filtered, and the solvent was removed in vacuo to yield13.0 g of yellow liquid. Benzaldehyde and benzyl alcohol were removed bydistillation under reduced pressure. The yellow solid was furtherpurified by column chromatography on silica gel with 1:1 hexanes/ethylacetate as the eluting system (Rf=0.2) to yield 2.8 g (52%) ofN,N-dibenzyl alanine. ¹ H NMR (300.15 MHz) δ1.39 (d, J=7.2 Hz, 3H), 3.54(q, J=8.7 Hz, 1H), 3.70 (ABq, δA=3.54, δ6=3.85, J=16.2 Hz, 4H),7.26-3.38 (m, 11H); HRMS for C₁₇ H₂₀ N₂ : 270.149. (calc'd 270.1489).

EXAMPLE 2 Preparation of α-(N,N-dibenzylamino)ethyl methyl ketone##STR7##

Triethylamine (8.85 mmol, 1.24 mL) was added under an argon atmosphereto a stirred solution of N,N-dibenzylalanine (8.85 mmol, 2.38 g) intetrahydrofuran (THF) (53.0 mL) at -30° C. (dry ice/CCl₄ bath).Trimethyl acetyl chloride (8.85 mmol, 1.42 mL) was then added dropwisevia syringe and the turbid solution was allowed to stir at -30° C. for30 minutes before dropwise addition of a solution of methylmagnesiumchloride in THF (3.0M, 1.07 eq., 9.5 mmol, 3.23 mL) over 10 minutes. Thesolution was stirred for 45 minutes and then quenched with saturatedsolution of NH₄ Cl (5.0 mL). The solution was extracted with Et₂ O, theorganic extracts washed with brine, dried over MgSO₄, and filtered. Thesolvent was evaporated in vacuo and the residue was subjected to flashcolumn chromatography on silica gel with 8:1 hexanes/ethyl acetate toisolate a pure product in 65% yield. Colorless oil; R_(f) =0.43 (7:1hexanes/ethyl acetate) ¹ H NMR (300.15 MHz) δ1.19 (d, J=5.4 Hz, 3H),2.25 (5, 3H), 3.39 (q, J=6.9 Hz, 1H), 2.57 (AB quartet, δ_(A) =3.33,δ_(B) =3. JAS=13.8 Hz, 4H), 7.19-7.43 (m, 10H); ¹³ C NMR (75.5 MHz 7.1,27.7, 54.6, 62.9, 27.2, 128.5, 128.8, 139.3, 210.8; IR (Neat) 1710 cm⁻¹; MS (low resolution) m/e 268 M+H 22%), 224 (MeCHNBn₂, 100%); HRMS forC₁₈ H₂₂ NO: 268-1709 (calc'd 268-1676); Anal: calc'd for C₁₈ H₂₁ NO: C80.68, H 7.92, N 5.24; Found: C 80.79, H 7.95, N 5.17.

EXAMPLE 3 Preparation of (5)-5-(N,N-dibenzylamino)-4-oxohexanoicacid-tert-butyl ester ##STR8##

Tetrahydrofuran (5.5 mL) was introduced via syringe to a three-necked 25mL round bottom flask purged with argon. The flask was cooled to -78° C.with a dry ice-acetone bath, and a solution of sodiumhexamethyldisilazide in THF (3.3 eq., 1.0M, 3.3 mL) was added. Compound2a (α-(N,N-dibenzylamino)ethyl methyl ketone, 1.0 eq., 3.0 mmol) in THF(3.5 mL) was then added dropwise via syringe. The resultant yellowsolution was allowed to stir at -78° C. for 1 hour, after which t-butylα-bromoacetate (3.1 mmol, 0.48 mL) was added neat to the enolatesolution. The reaction was quenched at -78° C. with a saturated solutionof NH₄ Cl after 30 minutes. The solution was extracted with Et₂ O (2×20mL), and the organic layer was separated and dried over anhydrous sodiumsulfate. The solvent was removed in vacuo and the resultant yellow oilwas then purified by flash column chromatography on silica gel with 8:1hexanes/ethyl acetate as the eluting system to obtain the product in 94%yield. Colorless oil, ¹ H NMR (300-15 MHz) δ1.22 (d, J=G-C¹ 3H), 1.44(s, 9H), 2.40-2.55 (m, 2H), 2.70-2.81 (m, 1H), 3.04-3.18 (m, 1H), 3-45(q, J=6-9 Hz, 1H), 3.62 (AB quo δ_(A) =3.49, δ_(B) =3.75, JAB=13.5 Hz,4H), 7.25-7.51 (m 10H); ¹³ C NMR (75.5 MHz) 7.1, 28.0, 29.4, 34.9, 54.5,62.2, 80.2, 127.1, 128.4, 128.7, 139.2, 171.9, 210.9; IR (Neat) 1720cm⁻¹ (br); MS (low res.) 382.4 (M+1, 21%), 326.3 (M - ^(t) Bu, 7%),224.3 (MeCHNBn₂, 100%); HRMS for C₂₄ H₃₂ NO₃ : 282.2373 (calc'd382.2374); Anal: calc'd for C₂₄ H₃₁ NO₃ : C 75.55, H 8-19, N 3.67, 012.58; Found: C 75-54, H 8.21, N 3.63.

EXAMPLE 4

Reduction of (5S)-5-(N,N-dibenzylamino)-4-oxohexanoic acid-tert-butylester (3a) to (5S)-5-(N,N-dibenzylamino)-44-hydroxy-hexanoicacid-tert-butyl ester (4a) ##STR9##

Sodium borohydride (2.0 eq., 3.3 mmol, 0.12 g) was added in one portionwith stirring to a solution of (5S)-5-(N,N-dibenzylamino)-4-oxo-hexanoicacid-tert-butyl ester (1.65 mmol, 0.63 g) in anhydrous methanol (20.0mL) at 0° C. (ice-water bath). The solution was stirred for two hoursand then quenched carefully with water (5.0 mL). Methanol was removed invacuo and the residue was extracted with Et₂ O, washed with brine, anddried over anhydrous Na₂ SO₄. The solvent was removed and the colorlessviscous oil obtained (crude wt.=0.65 g) was used in the reactiondescribed in Example 5 without further purification. ¹ H NMR (300.15)δ1.02 (d, J=6.9 Hz, 3H), 1.29 (s, 9H), 2.01-2.58 (m, 4H), 3.54 (ABquartet, δ_(A) =3.29, δ_(B) =3.80, JAB=13.2 Hz, 4H), 3.41-3.49 (m, 1H),4.45 (br, S, 1H), 7.22-7.35 (m, 10 H); MS (low res.) 384 (m+l, 18%), 328(M - ^(t) Bu, 11%), 224 (MeCHNBn₂, 100%); HRMS for C₂₄ H₃₄ NO₃ :384.2538 (calc'd 384.2530).

EXAMPLE 5

Cyclization of (5S) -5-(N,N-dibenzylamino)-4S-hydroxy-hexanoicacid-tert-butyl ester (4a) to (5S)-5[(1S)-1-(N,N-dibenzylamino)-ethyl]dihydrofuran-2(3H)-one (5a) ##STR10##

Glacial acetic acid (0.4 mL) and one drop of methanesulfonic acid wereadded to a solution of (5S)-5-(N,N-dibenzylamino)-4-hydroxy-hexanoicacid-tert-butyl ester (0.6 g, 1.57 mmol) in dry toluene (15.0 mL) in a50 mL round bottom flask fitted with a reflux condenser. The reactionmixture was heated at reflux for 12 hours and then cooled to ambienttemperature. The pH of the solution was adjusted to 7 by slow additionof a saturated solution of NaHCO₃. The organic layer was separated, andthe aqueous layer was extracted with Et₂ O (10 mL). The combined organicextracts were dried over MgSO₄, filtered, and the solvent was removed invacuo. The residue thus obtained was purified by flash columnchromatography on silica gel with 4:1 hexanes/ethyl acetate to obtain0.41 g (85% yield) of pure(5S)-5[(1S)-1-(N,N-dibenzylamino)-ethyl]-dihydrofuran-2(3H)-one (5a).White solid, mp. 61°-62° C., ¹ H NMR (300.15) δ1-11 (d, J=6.6 Hz, 3H),1.83-2.15 (m, 2H), 2.44-2.50 (m, 2 2.86 (pentet, J=6.9 Hz, 1H), 3.71(ABq, δ_(A) =3.87, δ_(B) =3.56, JAB=13.8 Hz, 4H), 4.49 (pentet, J=7.2Hz, 1H), 7.10-7.41 (m, 10H); ¹³ C NMR (75.8 MHz) 10.9, 25.6, 28.7, 54.3,55.8, 83.1, 126.8, 128.2, 128.7, 139.9, 177.1; MS (10W res.) 310 (M+1,20%), 224 (MeCHNBn₂, 40%), 91 (NBn₂, 100%); HRMS for C₂₀ H₂₄ NO₂ : 310.1807 (calc'd 310.1801); Anal calc'd for C₂₀ H₂₃ NO₂ : C 77.64, H 7.49, N4.53; Found: 77.54, H 7.55, N 4.49.

EXAMPLE 6

Alkylation of(5S)-5[(1S)-1-(N,N-dibenzylamino)ethyl]-dihydrofuran-2(3H)-one to(3S)-3-benzyl-(5S)-5-[(18)-1-(N,N-dibenzylamino)-ethyl]dihydrofuran-2(3H)-one (6a)##STR11##

To a stirred solution of sodium hexamethyldisilazide (0.27 mmol, 0.27mL) in 1.0 mL THF at -78° C., a solution of lactone 5a (0.25 mmol, 0.08g) in THF (0.5 mL) was added via a syringe under argon atmosphere. Thesolution was stirred at -78° C. for 1 hour and then treated with benzylbromide (0.27 mmol, 0.3 mL). The reaction mixture was quenched after 30minutes with saturated solution of NH₄ Cl. The organic layer wasseparated and the aqueous layer was extracted with Et₂ O (10 mL). Thecombined organic extracts were dried over MgSO₄, filtered and thesolvent was removed in vacuo. The residue was further purified by columnchromatography under standard conditions to give the alkylated lactone(3s)-3-benzyl-(5S)-5-[(1S)-1-(N,N-dibenzylamino)-ethyl]dihydrofuran-2(3H)-one(6a) in 81% yield (0.8 g). White solid, ¹ H NMR (300.15) δ1.05 (d, J=6.9Hz, 3H), 1.81-2.01 (m, 2H) 2.74-3.17 (m, 4H), 3.66 (AGq, δ_(A) =3.50,δ_(B) =3.82, JAB=13.8 Hz, 4H), 4.25 (q, J=6.6 Hz, 1H), 7.16-7.38 (m,15H); ¹³ C NMR 11.1, 30.4, 36.6, 41.2, 54.3, 55.6, 81.1, 126.7, 128.2,128.6=128.9, 138.2, 139.9, 178.7.

EXAMPLE 7

Ring Opening Amidation of (3S)-3-benzyl-(5S)-5-[(1S)-1-(N,N-dibenzylamino)-ethyl]-dihydrofuran-2(3H)-one (6a).

The amidation can be carried out as described in Basha, A., Lipton, M.,Weinreb, S. M., Tetrahedron Lett., 1977, 4171.

Examples 8 and 9 provide a detailed description for the preparation of(5S)-5-(N,N-dibenzylamino)-2-ethyl-4-oxohexano-(N,N-diethyl)-amide.

EXAMPLE 8 Preparation of 2-bromo-(N,N-diethyl)-butyramide ##STR12##

The synthesis of the 2-bromo-(N,N-diethyl)-butyramide can be carried outas described by Compagnone, R. S., and Rapoport, H. J. Org. Chem. 1986,51, 1713. Briefly, to a solution of (±)-2-bromobutyric acid (10.0 mmol,1.67 g) in THF (20.0 mL) at -10° C. under argon atmosphere was addedtriethylamine (10.7 mmol, 1.49 mL) followed by isobutyl chloroformate(10.7 mmol, 1.39 mL) dropwise. Diethylamine (12.5 mmol, 1.29 mL) wasadded to this suspension at -10° C. and the reaction mixture was stirredfor 40 minutes. The reaction was quenched by addition of 5.0 mL of a 5%solution of citric acid. The crude product was extracted with Et₂ O,washed with brine, dried over MgSO₄ and solvent evaporated in vacuo toprovide 2.4 g of residue which was purified by column chromatographyover silica gel with 4:1 hexanes-ethyl acetate as the solvent system(Rf=0.25). The product, 2-bromo-(N,N-diethyl)butyramide, was obtained in66% yield (1.46 g) as a colorless liquid. Analytical data: ¹ H NMR(300.15 MHz) δ0.90 (t, J=7.5 Hz, 3H), 1.05 (t, J=7.2 Hz, 3H), 1.16 (t,J=7.2 Hz, 3H), 1.85-2.21 (m, 2H), 3.08-3.52 (m, 4H), 4.20 (t, J=7.2 Hz,1H); ¹³ C NMR (75.5 MHz) 12.1, 12.3, 14.7, 28.3, 40.8, 42.2, 45.5,167.8; IR (neat) 1661 cm⁻¹ (amide C═O str.)

EXAMPLE 9 Preparation of(N,N-diethyl)-(5S)-5-(N,N-dibenzylamino)-2-ethyl-4-oxo-hexanamide##STR13##

To a solution of NaHMDS (0.52 mmol, 0.53 mL) in 1.0 mL THF at -78° C.was added a solution of α-(N,N-dibenzylamino)ethyl methyl ketone (0.5mmol, 0.13 g) in 0.5 mL THF under argon atmosphere. After stirring theyellow colored solution for one hour, a solution of2-bromo-(N,N-diethyl)-butyramide (0.51 mmol, 0.11 g) in 0.5 mL THF wasadded dropwise. The solution was stirred at -78° C. for 30 minutes andthen at ambient temperature for another two hours before quenching with2.0 mL of saturated solution of NH₄ Cl. After a work-up as described inExample 8, the residue was purified by column chromatography over silicagel with 4:1 hexanes-ethyl acetate as the solvent system (Rf=0.21) togive 0.03 g of α-(N,N-dibenzylamino)ethyl methyl ketone, the startingketone, and the product as a mixture of diastereomers (0.082 g, 55%yield based on recovered starting material) as a colorless oil.Analytical data: ¹ H NMR (Very complex spectrum due to diastereomers,diastereotopic protons and rotamers due to rotation around the N--C═Obond); ¹³ C NMR (one isolated diastereomer) 7.1, 11.7, 12.9, 14.4, 25.9,37.6, 40.2, 41.9, 42.5, 54.6, 62.1, 127.1, 128.4, 128.6, 139.2 (C═O andN--C═O were not observed possibly due to the low concentration of thecompound); IR (neat) 1720 (s, C═O str. for ketone), 1640 (s, C═O str.for amide) cm⁻¹. Mass spectrum (low resolution) 409 (M+1, 30%), 224(M-MeCHNBn₂, 100%). HRMS for C₂₆ H₃₇ N₂ O₂ : 409.284 (calc'd 409.2846)

Examples 10 and 11 provide a detailed description for the preparation of(5S)-5-(N,N-dibenzylamino)-2-ethyl-4-oxo-hexanoic acid-tert-butyl esterfrom tert-butyl-2-bromobutyrate and α-(N,N-dibenzylamino)ethyl methylketone. This compound is reduced to the corresponding amino alcohol asdescribed in detail above.

EXAMPLE 10 Preparation of tert-butyl-2-bromobutyrate ##STR14##

tert-Butyl-2-bromobutyrate was synthesized according to the proceduredescribed in Compagnone, R. S., and Rapoport, H. J. Org. Chem. 1986, 51,1713.

EXAMPLE 11 Preparation of(5S)-5-(N,N-dibenzylamino)-2-ethyl-4-oxo-hexanoic acid-tert-butyl ester##STR15##

A solution of α-(N,N-dibenzylamino)ethyl methyl ketone (0.5 mmol, 0.13g) in 0.5 mL THF was added to a solution of NaHMDS (0.53 mmol, 0.53 mL)in 1.0 mL THF at -78° C. under argon atmosphere. After stirring theyellow colored solution for one hour, a solution oftert-butyl-2-bromobutyrate (0.51 mmol, 0.11 g) in 0.5 mL THF was addeddropwise. The solution was stirred at -78° C. for 30 minutes, and thenat ambient temperature for another 2 hours, before quenching with 2.0 mLof saturated solution of NH₄ Cl. After a work-up as described in Example7, the residue was purified by column chromatography over silica gelwith 4:1 hexanes-ethyl acetate as the solvent system (Rf=0.53) to givethe product as a mixture of diastereomers (0.047 g, 23% yield) as acolorless oil. Analytical data: ¹ H NMR (300.15 MHz) δ0.83 (t, J=7.5 Hz,3H), 1.22-1.72 (m, 8H) 1.39 (s, 9H), 3.62-3.86 (m, 5H), 7.23-7.33 (m,10H); Mass spectrum (low resolution) 410 (M+1, 30%), 224 (M-MeCHNBn₂,100%). HRMS for C₂₆ H₃₅ O₃ NLi: 416.2773 (calc'd 416-2768).

Examples 12 and 13 provide a detailed description for the preparation ofmethyl-(5S)-5-(N,N-dibenzylamino)-4-oxo-hexanoate from1-bromo-(3S)-3-N,N-dibenzylamino-butan-2-one.

EXAMPLE 12 Preparation of 1-bromo-(3S)-3-N,N-dibenzylamino-butan-2-one##STR16##

A solution of LDA was generated by addition of n-butyllithium (1.05mmol, 2.5M) to freshly distilled diisopropylamine (1.2 mmol, 0.16 mL) in2.0 mL THF at -78° C. under argon atmosphere. To this solution was addeda solution of α-S-(N,N-dibenzylamino)ethyl methyl ketone (1.0 mmol, 0.13g) in 1.0 mL THF. The resulting solution was stirred at -78° C. for onehour and then a solution of bromine (1.05 mmol, 0.06 mL) in 0.5 mL CH₂Cl₂ was introduced dropwise. The reaction mixture was stirred at -78° C.for 10 minutes and then quenched with saturated solution of NaHCO₃. Theorganic layer was extracted with pentane (2×15.0 mL), washed with brine,dried over MgSO₄, and filtered. The solvent was removed in vacuo toobtain a yellow-orange colored oil which solidified on standing. Thecrude ¹ H NMR showed presence of starting material (approximately 5%),but the reaction product was used in the reaction described in Example13 without further purification. Analytical data: ¹ H NMR (300.15 MHz)δ1.21 (d, J-6.6 Hz, 3H), 3.60 (AB_(q), δ_(A) =3.43, δ_(B) =3.69, J=13.5Hz, 4H), 3.68 (q, J=6.6 Hz, 1H), 4.17 (AB_(q), δ_(A) =4.14, δ_(B) =4.19,J=13.5 Hz, 2H), 7.23-7.37 (m, 10H).

EXAMPLE 13 Preparation ofmethyl-(5S)-5-(N,N-dibenzylamino)-4-oxo-hexanoate ##STR17##

Methyl acetate (1.0 mmol, 0.08 mL) was added dropwise to a solution ofLDA (1.05 mmol) in 2.0 mL THF at -78° C., and the resulting solution wasstirred for 45 minutes. Hexamethyl phosphoric triamide (1.0 mmol, 0.18mL) was then added, and after 15 minutes a solution ofα-(N,N-dibenzylamino)ethyl bromomethyl ketone (crude from the previousreaction, assumed to be 1.0 mmol) in 2.0 mL was added via syringe. Theresulting solution was stirred at -78° C. for 45 minutes and thenquenched with saturated solution of NaHCO₃. After a work-up as describedin Example 8, 0.39 g of crude product was obtained. The product wasfurther purified by column chromatography on silica gel with 8:1hexanes-ethyl acetate as the solvent system (Rf=0.33) to yield 0.21 g(62%) of methyl-(5S)-5-(N,N-dibenzylamino)-4-oxo-hexanoate as acolorless viscous oil. Analytical data: ¹ H NMR (300.15 MHz) δ0.95 (d,J=6.6 Hz, 3H), 2.62-2.30 (m, 4H), 3.36 (q, J=6.2 Hz, 1H), 3.60 (AB_(q),δ_(A) =3.41, δ_(B) =3.82, J=13.5 HZ, 4H), 3.49 (S, 3H), 7.23-7.37 (m,10H); ¹³ C NMR 5.1, 38.9, 48.8, 51.4, 54.4, 57.5, 126.9, 128.2, 128.8,129.2, 139.4, 170.4, 208.2.

This invention has been described with reference to its preferredembodiments. Variations and modifications of the invention will beobvious to those skilled in the art from the foregoing detaileddescription of the invention. It is intended that all of thesevariations and modifications be included within the scope of theappended claims.

We claim:
 1. A process for the preparation of a (4S,5S)-hydroxyethylenedipeptide isostere comprising the step of reacting the enolate ofα-N,N-di(protected)amino methyl ketone with a compound selected from thegroup consisting of α-(leaving group)-acetate and α-(leavinggroup)-acetamide, wherein the amino protecting groups are capable ofcontrolling the facial selectivity of reaction of the enolate.
 2. Theprocess of claim 1, wherein the α-N,N-di(protected)amino methyl ketoneis R¹ CHN(R²)₂ C(O)CH₃, wherein:R¹ is selected from the group consistingof the residue of an amino acid, alkyl, aryl, alkaryl, heteroaromatic,alkyl(heteroaromatic), arylamino, and aralkyl groups; and both R²moieties are benzyl or one R² moiety is benzyl and the other is selectedfrom the group consisting of t-butyloxycarbonyl, benzyloxycarbonyl, andp-methoxybenzyl.
 3. The process of claim 2, wherein R¹ is selected fromthe group consisting of methyl, ethyl, benzyl, hydrogen, isopropyl,HOCH₂ --, --CHOHCH₃, --CH₂ SH, 2-methylpropyl, 1-methylpropyl, --CH₂ CH₂SCH₃, --CH₂ (indole), --CH₂ (p-hydroxyphenyl), --(CH₂)₄ NH₂, --CH₂(imidazole), --CH₂ CH₂ C(O)NH₂, --CH₂ C(O)NH₂, --CH₂ CO₂ H, and --CH₂CH₂ CO₂ H, which is optionally protected during the process.
 4. Theprocess of claim 1, wherein the α-(leaving group)acetate is selectedfrom the group consisting of CH₂ XC(O)OR³ and R⁴ CHXC(O)OR³, whereinR³is selected from the group consisting of alkyl, aryl, heteroaromatic,alkylheteroaromatic, aralkyl, and alkaryl; R⁴ is selected from the groupconsisting of alkyl, alkaryl, aryl, aralkyl, heteroaromatic,alkylheteroaromatic; and X is selected from the group consisting ofbromo, chloro, iodo, triflate, tosylate, diazonium salt, mesylate, andbrosylate.
 5. The process of claim 4, wherein R³ is t-butyl.
 6. Theprocess of claim 4, wherein R⁴ is selected from the group consisting ofmethyl, ethyl, propyl, isopropyl, butyl, isobutyl, cyclohexylmethyl,benzyl, phenethyl, and t-butyl.
 7. The process of claim 4, wherein X isbromo.
 8. The process of claim 2 wherein both R² moieties are benzyl. 9.The process of claim 1 wherein the α-(leaving group)-acetamide isselected from the group consisting of CH₂ XC(O)N(R⁶)₂ and R⁴CHXC(O)N(R⁶)₂, wherein R⁶ is independently hydrogen, alkyl alkaryl,aryl, aralkyl, heterocyclic, or alkylheterocyclic.
 10. The process ofclaim 2, wherein R¹ is selected from the group consisting of methyl,benzyl, and cyclohexylmethyl.
 11. The process of claim 1, furthercomprising reducing the ketone and then cyclizing the hydroxyethylenedipeptide product to corresponding lactone.
 12. The process of claim 11or 30, wherein the lactone is of the structure: ##STR18##
 13. Theprocess of claim 12, further comprising adding an R⁵ group to thelactone by reacting the enolate of the lactone with XR⁵,wherein X isselected from the group consisting of bromo, chloro, iodo, triflate,tosylate, diazonium salt, mesylate, and brosylate, and R⁵ is selectedfrom the group consisting of primary and secondary alkyl and benzyl, toproduce a compound of the structure: ##STR19##
 14. The process of claim13, wherein R⁵ is selected from the group consisting of methyl, benzyl,isobutyl, isopropyl, and cyclohexylmethyl.
 15. The process of claim 13,further comprising opening the lactone to the corresponding amide,ester, or carboxylic acid.
 16. The process of claim 1, wherein R¹CHXC(O)OR³ or R⁴ CHXC(O)N(R⁶)₂ is reacted with R¹ CHN(R²)₂ C(O)CH₃ toform R¹ CHN(R²)₂ C(O)CH₂ CHR⁴ C(O)OR³, whereinR¹ is selected from thegroup consisting of the residue of an amino acid, alkyl, aryl, alkaryl,heteroaromatic, alkyl(heteroaromatic), arylamino, and aralkyl groups;both R² moieties are benzyl or one R² moiety is benzyl and the other isselected from the group consisting of t-butyloxycarbonyl,benzyloxycarbonyl, and p-methoxybenzyl; R³ is selected from the groupconsisting of alkyl, aryl, heteroaromatic, alkylheteroaromatic, aralkyl,and alkaryl; R⁴ is selected from the group consisting of alkyl, alkaryl,aryl, aralkyl, heteroaromatic, alkylheteroaromatic; R⁶ is independentlyhydrogen, alkyl, alkaryl, aryl, aralkyl, heterocyclic, oralkylheterocyclic; and X is selected from the group consisting of bromo,chloro, iodo, triflate, tosylate, diazonium salt, mesylate, andbrosylate.
 17. The process of claim 16 or 31, further comprisingreducing the ketone in R¹ CHN(R²)₂ C(O)CH₂ CHR⁴ C(O)OR³ and thencyclizing the reduction product to the corresponding lactone.
 18. Theprocess of claim 17, further comprising adding an R⁵ group to thelactone by reacting the enolate of the lactone with XR⁵,wherein X isselected from the group consisting of bromo, chloro, iodo, trillate,tosylate, diazonium salt, mesylate, and brosylate, and R⁵ is selectedfrom the group consisting of primary and secondary alkyl and benzyl, toproduce a compound of the structure: ##STR20##
 19. The process of claim18, further comprising opening the lactone to the correspondingcarboxylic acid or amide.
 20. The process of claim 1, wherein theisostere is(N,N-diethyl)-(5S)-5-(N,N-dibenzylamino)-2-ethyl-4-oxo-hexanamide. 21.The process of claim 1, wherein the isostere is(N,N-dialkyl)-(5S)-5-(N,N-dibenzylamino)-2-alkyl-4-oxo-alkylamide.